1. Field of the Invention
The present invention relates to an aging device which is an electronic timer free from a battery.
2. Description of the Related Art
Recently, an electronic timer free from a battery, that is, an aging device in which output is varied as time passes is beginning to be used in the fields of content delivery, credit cards, rental of digital products, rental of digital software, and so on.
The aging device has a floating gate and operates as an electronic timer for measuring a certain period by using retention characteristics (life) of a charge in the floating gate (for example, see Jpn. Pat. Appln. KOKAI Publication Nos. 2004-172404 and 2005-310824).
According to the above aging device, for instance, the amount of the charge injected into the floating gate and the thickness of a tunnel insulating film are changed, whereby the life of the aging device can be changed.
There are two types of the aging devices: stack gate type and single-layer polysilicon type.
Since a basic structure of the stack gate type is the same as the basic structure of a memory cell of a flash memory, it is preferable that the stack gate type is adopted when the aging device is mounted on an IC with a built-in flash memory.
However, the aging device is also employed in an IC without a flash memory. In this case, it is preferable that the single-layer polysilicon type is adopted.
Jpn. Pat. Appln. KOKAI Publication No. 2007-184321 proposes the single-layer polysilicon type.
This single-layer polysilicon type is characterized in that a writing/erasing part (second element region) is provided in addition to an aging device part (first element region). In addition, a floating gate is disposed so as to cross over both the aging device part and the writing/erasing part.
Thus, a coupling capacitance between the floating gate and the first element region is rendered larger than a coupling capacitance between the floating gate and the second element region, whereby the charge is easily injected and ejected between the writing/erasing part (second element region) and the floating gate.
In the above single-layer polysilicon type, all the following patterns are required to be realized. This is because four basic operations, which are features of the aging device disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-172404, can be realized.
[N-Channel]
Normally-On:
writing (injection of electrons to the floating gate) is performed at initialization. The aging device is off in a writing state (initial state), and on after a lapse of the life time.
Normally-Off:
erasing (ejection of electrons from the floating gate) is performed at initialization. The aging device is on in an erasing state (initial state), and off after a lapse of the life time.
[P-Channel]
Normally-On:
erasing (ejection of electrons from the floating gate) is performed at initialization. The aging device is off in the erasing state (initial state), and on after a lapse of the life time.
Normally-Off:
writing (injection of electrons to the floating gate) is performed at initialization. The aging device is on in the writing state (initial state), and off after a lapse of the life time.
In order to realize the normally-on in the N-channel type, positive high voltage is applied to an N-type diffusion layer in the first element region, which is the aging device part, in the writing. In order to improve the writing speed, although negative high voltage is also preferably applied to the N-type diffusion layer in the second element region as the writing/erasing part, it is difficult to simultaneously apply the positive and negative high voltages to the same semiconductor substrate.
In addition, in order to realize the normally-off in the N-channel type, the positive high voltage is applied to the N-type diffusion layer in the second element region, which is the writing/erasing part, in the erasing. In order to improve the erasing speed, although the negative high voltage is also preferably applied to the N-type diffusion layer in the first element region as the aging device part, it is difficult to do that for the reason described above.
Further, the same can be said for the case of realizing the normally-on and the normally-off in the P-channel type.
As above described, there is a problem that the speed of writing at initialization is not sufficiently increased in the prior art single-layer polysilicon type aging device.